The present invention relates to systems (methods and apparatus) for testing and measuring the performance of CT and MRI imaging systems, and to test targets for CT and MRI imaging, which are often called phantoms, and enable the assessment of the performance of the imaging system in terms of criteria that modern imaging science has recognized as necessary or desirable for such assessment including, for example, the modulation transfer function of the system, resolution, aliasing, and spatial frequency response.
CT imaging systems generate and scan x-ray beams while MRI systems obtain image information from a preselected tomographic slice or cross section of thickness typically greater than one mm. Sometimes the image signals are transmitted over communications links to a receiving station far removed from the patient where diagnoses based upon the images are made. It is important for medical imaging that the quality of the entire system be assessed. It therefore is desirable to have test targets or phantoms which provide images from which the performance of the entire system can be assessed.
Imaging science has developed criteria, such as the modulation transfer function (MTF), which can provide assessment of aliasing, spatial frequency response, and resolution limits for the evaluation of imaging systems generally. Such assessments have not been feasible with many conventional phantoms. These phantoms use objects which mimic lesions of different size and contrast. Rods, spheres, cones and other geometrical objects of a size which can be resolved by the system are used in conventional phantoms and are located in volumes containing water or tissue mimicking material (such as gels). None of these phantoms are able to produce detailed, high resolution patterns at varying contrast levels that permit accurate evaluation of MTF and other imaging science criteria which represent the performance of the imaging system. Some phantoms have been suggested which use solid bars in three-dimensional space, but these phantoms have not been provided with precisely defined patterns from which imaging science criteria can be determined.
The present invention provides a system (method and apparatus) for evaluation and assessment of MRI and CT imaging systems and improved phantoms which can test the overall response and performance of the imaging system thereby revealing system performance with imaging science criteria, such as MTF and similar analytical assessments. The combined influence of all stages in the imaging system including any communication link, which is used for teleradiography, may thereby be evaluated. Further information concerning imaging science criteria may be had from C. R. Hill et al, Ultrasound in Mod. and Biol. 17, 6, 559, and A. Rose et al, Physics Today, September 1989, P.24-32.
Phantoms can be provided in accordance with the invention on thin films or sheets by conventional printing techniques, such as electrostatic or xerographic printing, as with a laser printer, thereby providing regions with precise control in local concentration, as well as distribution, of scatterers. These are regions of sub-resolvable size (micro-regions) with preselected magnetic resonance or x-ray absorption properties. The scatterers are of sub-resolvable size which is less than the resolution voxel (a three-dimensional volume element-viz. a 3-D pixel). Because of the thin substrate, in the form of a planar medium on which the regions are located, tomographic imaging of the entire pattern is facilitated. The image brightness and contrast can be precisely controlled in the formation of the regions thereby providing precisely controlled MRI and CT signals, both from the regions and their positions on the substrate (the patterns of the regions). Since the actual scatterers are sub-resolvable, the imaging system can only detect their aggregate presence or absence, not the exact number and exact position of individual scatterers. The individual scatterers may be referred to as xe2x80x9cdigitalxe2x80x9d (either on or off, there or not there) in nature. The precise placement of these digital scatterers can then be used to create regions of controllable MRI or CT signal strength based on their number per unit area and their arrangement relative to each other, similar in concept to a half-tone printing process. These patterns of regions can be analyzed with the same computer system algorithms as used in conventional optics imaging systems, thus, facilitating the measurement of the imaging science criteria, such as the Modulation Transfer Function (MTF) which is defined as the normalized ratio of the measured intensity modulation of an image relative to the known intensity modulation of the originating object as a function of spatial frequency. Intensity modulation is defined as the ratio of maximum intensity difference to the sum of the intensity level extremes Imax and Imin, i.e. Modulation=            I      ⁢              xe2x80x83            ⁢      max        -          I      ⁢              xe2x80x83            ⁢      min                  I      ⁢              xe2x80x83            ⁢      max        +          I      ⁢              xe2x80x83            ⁢      max      
We have discovered that the aforementioned thin film phantom with subresolvable, digital scatterers, can also be employed to produce useful imaging science test patters for magnetic resonance imaging (MRI) and x-ray computed tomography (CT) systems. In MRI, cross-sectional images are produced whereby the image intensity depends on a number of factors including the local material magnetic properties, proton density, and relaxation constants T1 and T2 (See, Foundations of Medical Imaging, Cho, Jones and Singh, Wiley and Sons, NY 1993). Phantoms have been constructed using various plates, tubes, and regions that are filled with paramagnetic materials or simply materials with different proton density and relaxation constants in order to produce test patterns in an MRI image (See, U.S. Pat. Nos. 4,692,704, J. Grey, September 1987; 4,625,168, Meyer et. al, November 1986.) In CT imaging, image brightness is dependent on a number of factors including the x-ray attenuation coefficient of the materials within the imaging cross-section. CT phantoms have been constructed using various plates, tubes, and regions that possess different x-ray absorption coefficients so as to produce a pattern on the CT cross-sectional image.
The invention provides a thin-film phantom with digital scatterers in predetermined patterns. Such patterns may take the form of half-tone masks for grey scale contrast evaluation. They may be in the form of chirp and other suitable patterns for MTF and other system response criteria determination.
While electrostatic or xerographic printing is presently preferred, other techniques for making patterns on thin-film substrates including lithography, sputtering, vacuum deposition and etching may be used. The scatterers in the region are of dimensions much finer than that of a resolution voxel produced by the MRI and CT imaging system. For example, for a diagnostic MRI system using a 1.5 Tesla magnet and body coils, a 1 mmxc3x971 mm by 3 mm slice thickness is a type of resolution voxel. A conventional 300 dots per inch laser printer using 10 micron toner particles can produce scatterers having a size of approximately 85 microns which are sub-resolvable in terms of the imaging system resolution. Thus, patterns of regions of sub-resolvable scatterers can be provided on a thin film substrate to afford phantoms for testing for different criteria. The patterns may be regularized or periodic profiles. Bars generated by a half-tones screen or mask, preferably a blue noise mask may be used. See Parker et al, U.S. Pat. No. 5,111,310 issued May 5, 1992 for information concerning blue noise mask generation by computer techniques.
As in the case of ultrasound phantoms of the parent application referenced above, the thin-film phantoms may be precisely displaced, preferably by utilizing a piezo electric material, such as PVDF as the substrate, across which a varying electrical field is applied by means of electrodes. The field may be sinusoidal to set up sinusoidal vibrations or may use other waveforms for other displacement characteristics which may be desired. For example, electrical waveforms such as ramps, chirps, AM signals, FM signals and even musical tones may be applied to induce displacements and produce Doppler signals in the audio range. The displacements are preferably at a vibration rate which should be less than the pulse repetition rate (PRF) of the pulses which comprise the interrogating beam and a size which maintains the sub-resolution characteristics of the scatterers in the regions.
Briefly described therefore, the invention provides a system for testing MRI and CT imaging systems which scan using electromagnetic energy and x-ray beams. The scan is over a plane to form an image of (para) magnetic or energy-absorbing objects in the plane. In accordance with the invention, different patterns of regions of sub-resolvable scatterers on planar mediums are provided. The mediums are of such magnetic resonance or x-ray absorption characteristics that scanning occurs in and along the plane of the medium without resonance or absorption effects. The image formed by the interaction with the pattern is analyzed to evaluate the performance of the system, preferably utilizing imaging science criteria.